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Winterizing a seasonal outdoor pool can vary greatly depending upon location, pool type, groundwater conditions, local sewer capacities and a host of other factors. If an outdoor swimming pool is not properly winterized, catastrophic damage to below grade piping, heaving and floating of the pool tank, undue wear on pool mechanical equipment, delamination of finishes and numerous other physical and cosmetic issues will can occur. And when winterization is required, there are traditionally three (3) options for consideration regarding whether or not water is remains within the pool tank. These options include leaving the pool water at or near the normal operating level, a partial draining of the pool, or a complete removal of all pool water. Regardless of the method that is used there are several considerations that should not be overlooked.

In almost all geographic regions there are will be variations in outside air temperature and seasonal precipitation that will result in varying hydraulic forces from both the surface (water within the pool basis) as well as potential upward forces from fluctuations in groundwater levels. In climates that experience extended periods of time where the temperature remains below freezing, these problems are complicated by the freeze-thaw cycle. Water trapped in porous areas of concrete or behind ceramic tile or leaking through improperly caulked expansion joints may cause damage at the first hard freeze. Frost heave describes the lifting or displacing force of freezing water. If the pool is to be drained and left empty, the surrounding soil must be well drained and dry. Even amounts of water too small to cause hydrostatic damage can cause frost heave, and this lifting or movement can crack concrete slabs or displace sections of the pool structure or deck.

High Groundwater Table Considerations:

In situations where the level of groundwater is high, the force exerted on the pool bottom and sidewalls can result in cracking, differential movement or other structural damages. In the most severe conditions where groundwater level are noticeably above the pool bottom, portions of the structure can be lifted or moved by the buoyant forces. If circumstances require the pool to be drained, the groundwater level should first be determined, and confirmation that hydrostatic relief valves are present and functional should be completed. Hydrostatic relief valves allow groundwater to flow into the empty pool when the surrounding water table is too high thus providing a ballast to the upward buoyant pressure. This valve functions as a check to prevent the flow of water from the pool into the ground when the pool is full.

Swimming pools located in areas with high water tables often have systems of perforated pipes or under-drains that allow water to flow away from the pool. These systems either carry the water into porous soil areas adjacent to the pool or allow drainage by gravity. For the gravity method to succeed, considerable changes in ground level must be present, as the pipe must go to “daylight” at an elevation several feet below the bottom of the pool. In some instances, these drainage networks connect to a sump or pit where the groundwater level can be observed and removed via a sump pump.

Pool Draining:

For emptying the pool, a quick and efficient means provided via gravity is almost always preferred. If gravity is not possible, a filter bypass line can be designed on the discharge side of the recirculation pump and piped to the backwash pit. A 3” or 4” line will typically be used with a normally closed butterfly valve located on the drain line. The difference in the recirculation rate and what the sewer can handle can be sent back to the pool through the filters. A small feature pump could also double as a drain down pump. The pump would need to pull from the deep main drain and the project engineer should confirm the pump size with the sewer capacity.

The filter recirculating pump may not be able to pump the pool completely empty because of capacity discrepancies between the pump and the sewer, as well as vortexing concerns at the main drain outlets. For this reason, occasionally a small low head pump is provided for drawing the final water out of the pool. The suction for this pump is located at the bottom of the deepest main outlet sump.

Lastly when winterization protocol requires the pool tank to have water remain, the best and most appropriate off-season protection for both pool and people is a safety cover. Such a cover protects the pool from debris, greatly reduces the growth of algae, and protects children and animals from falling in the pool.

There are four rationales for leaving water in a pool in the off season:

It is a way to winterize the floor inlet system.

It will stabilize the pool shell should the ground water table rise during the spring and hydrostat malfunctions.

It protects the pool floor surface from spalling due to sudden changes in temperature over numerous seasons.

It insulates the pool floor and thus the substrate and the soil beneath. If the soil freezes, the floor can heave creating cracks and possible loss of water.

Winterization Items beyond the Pool Tank:

Thoroughly cleaning and lubricating, coating or protecting virtually everything in the swimming pool vicinity can help stave off corrosion during the off-season. Equipment must be adequately winterized, dismantled, or removed and stored. Chemicals must be properly removed – not simply left sitting for the next season, dumped into the pool or poured down the drain.

If the pumps, hair and lint pot, filter, and heater are not drained, they can freeze, crack and have to be replaced. Likewise, chemical feeders may be damaged. In extreme cold weather climates providing temperament heat within the pool filter building is a good recommendation.

An outdoor pool in a freezing climate must have all water removed from its pipes before a hard freeze occurs. Therefore, blow out all pipes by means of an air blower and a winterization tap, and then cap all pipes. For added protection against freezing pipes, the pipes can be filled with antifreeze. A ½” hole should be tapped in the piping after the check valve on pressure piping and plugged, or a 1” pipe with a ball valve can run to the nearest pump pit drain. Either option should allow the pipe to be drained. All of the piping needs to drain to the deep main drain to help eliminate low spots and freezing.

Hidden Concerns:

A pool cannot winterize itself like a self-cleaning oven or an auto defrost freezer. Therefore, thorough detail must be followed during your winterization process. For instances where the pool is to be completely drained, all too common a complete and sufficient draining and subsequent blowing air through the pipes is not completed. Water trapped in a 45 or 90 degree fitting will break first as compared to straight pipe because the ice is constructed when it begins to expand. As compared to a vertical or horizontal run of pipe water will freeze and expand without breaking the pipe.

If water is left in the pool, ice can damage the water-line tile, or worse yet, the gutter tile. Ice can also damage the underwater lights if the water level reaches the level of the lights due to evaporation, a leak at a light niche or due to unusual rain. A winterizing option is to remove the light from the wet niche, place it in a plastic bag with a brick and sink the light to the bottom of the pool, which will keep the fixture gasket wet and avoid drying it out which will result in the gasket leaking at the start of the season.

Waterslides and recreational water features will constantly expand and contract with extreme variations in temperature. Joints between slide flume sections will need to be caulked as thermal expansion and contraction leads to leaking slide joints.

Lasting Recommendations:

The pool should be engineered so the operator can winterize the pools easily and thoroughly. Design the piping system so water will drain by gravity to daylight, i.e. filter room, surge pit or nearby manhole is ideal. All of these sites will have dedicated drain pipes with on/off valves. If draining to daylight is not possible, provide galvanized female threaded fittings on the pressure and suction lines in the filter room for convenient use with a compressor. With the compressor, blow the respective lines with propylene glycol antifreeze. The antifreeze will mix with any residual water in the lines and thereby lower the freeze point of the mixed liquid in any undulation in the underground piping.

If the pool must be empty for the winter, drain the pool, winterize all of the lines with a compressor and antifreeze, and pump the main outlet pipes dry with a sump pump in the outlet sumps. Once the pool piping has been drained plug all lines with expandable rubber plugs. Plug all inlets and pipe openings in skimmers. Always remember to confirm that all hydrostatic valves are operable.

If the owner permits the pool to be half full of water during the winter, then the floor inlet piping will not need to be winterized. The water depth will preclude freezing to shallow end floor and thus below pool lines will be protected. However it is possible that if some water is left in the pool, the water level may change over the course of a winter via evaporation or precipitation causing unforeseen issues. Therefore, a consistent protocol for monitoring the pool must be done.

Overall, winterization of the seasonal commercial pool should not be considered a daunting task. As long as a consistent process is in place resulting in a complete and thorough winterization, your outdoor pool should have the ability to withstand even the harshest of winter conditions. However, without a good winterization process, your pool may experience premature aging and unforeseen repairs.